This invention relates to a vehicle communication system comprising a number of radio zones.
In a mobile communication system in which a service area is constituted by a plurality of radio zones each installed with a base station provided with a plurality of channels, a common control channel system is generally used including a mobile paging channel (P-CH) and a mobile access channel (A-CH) which are used to establish a speech channel (S-CH) so as to decrease the set-up time of a call connection and to efficiently utilize frequencies.
As disclosed on pages 1172-1190 of "Review of Electrical Communication Labaratories" Vol. 25, Nos. 11-12, 1977 published by Nippon Telegraph and Telephone Public Corporation, when making a mobile terminating call through a paging channel (P-CH), a page code is sent from a control center to a mobile. Then the mobile sends out a page response code in response to the page code. In a case shown in FIG. 1, a control center CC sends the page code to both base stations BS1 and BS2. Where a mobile is in a radio zone of the base station BS1, the page response code is sent back to the control center CC via the base station BS1. Since the control center receives the page response code from the mobile by way of the base station BS1, it sends an idle channel number among a number of channels allocated to the base station BS1 by means of a channel assign code to the mobile. In the mobile, the channel is skipped to the assigned channel, and after checking the loop a bell is rung. Then the mobile subscriber station (MSS) permits a mobile subscriber to begin talking. In this system, many mobiles utilize the paging channel P-CH on the time division basis to improve the utilization efficiency.
As shown in FIG. 2, the access channel is used at the time of a mobile originating call. More particularly, in the mobile, the channel is changed from the paging channel P-CH to the access channel A-CH through which an access-channel-mark code is sent out. Since the control center CC receives the origination code from the base station BS1, it selects an idle channel out of a number of channels allocated to the base station BS1 for sending a channel assign code to the mobile through the access channel. The mobile skips to the assigned channel to make a loop check and sends out a dial code to begin talking.
Reservation of an access channel is necessary because where an idle channel among a number of speech channels is selected to transmit an origination code therethrough and then utilize the selected channel as a speech channel it takes a long time to search the idle channel. Where the access channel is fixed, however, it can not convey as much traffic as that of the paging channel because individual mobiles simultaneously send origination codes at a high probability, resulting in a so-called call collision.
In a most widely used vehicle communication system, as the channel exclusively used as a paging channel was provided for each base station a single channel having a predetermined frequency common to all zones, whereas as control channels exclusively used for accessing were provided for each base station one or more channels having predetermined frequencies common to all zones, the number of channels being determined by a maximum expectant number of the mobiles moving into or out of the service area of each base station.
More particularly, the frequency of the radio carrier signal and the phase of the modulated signal are adjusted in an overlapped zone to minimize interference between these signals. Thus, as shown in FIG. 3 in radio zones I and II, a frequency f1 is allocated to the paging channel P-CH and frequencies f2 and f3 to the access channel A-CH.
In the mobile communication system described above, there is no problem in the mobile paging channel, but in the case of the mobile access channel, as it utilizes specific frequencies common to all zones, the number of transceivers in each base station increases greatly, thus increasing the cost of installation as will be described later in more detail. On the other hand, when the number of the transceivers is fixed, the number of the speech channels decreases proportionally.
From the standpoint of the increase in the number of the speech channels where the traffic is busy, it is desirable to use the mobile access channel as a speech channel.